This invention relates to a device for defibering fibrous material. More particularly this invention relates to a device in grinding apparatus for lignocellulose containing material which have grinding discs housed rotatably relatively to one another in a pressurized grinding casing, the surfaces of said grinding discs facing one another and defining therebetween a grinding interspace into which the stock to be ground is fed through a central supply passage. A pressurized gaseous medium such as steam or other treating gas is present in the interspace or generated therein during the grinding process with consequent higher pressure than in the passage. The device according to the invention can also be used where initially produced fiber pulps are treated in a subsequent grinding process to desired processing degree.